1. Field of the Invention
The present invention relates to an imaging device and a biometrics authentication apparatus.
2. Description of Related Art
Biometric authentication technology using human body characteristics such as fingerprints, irises and blood vessel patterns receives attention as a new security system. The biometric authentication technology offers convenience because there is no need to take along unlike a key or the like. Further, because there is also no possibility of loss unlike a key or the like, the risk of a threat to safety and security due to theft or the like is low.
Particularly, an authentication method using blood vessel patterns in a human finger or the like is not associated with criminal investigation unlike the method using fingerprints and does not require the application of light to eyeballs unlike the method using irises. Thus, the use of blood vessel patterns reduces the possibility that a person who undergoes the authentication process feels psychologically stressed. Further, because the authentication method using blood vessel patterns utilizes the information in vivo, the possibility of forgery is low.
The biometric authentication using a blood vessel pattern in a human finger is under development. Typically, a light source which applies a near-infrared ray to a finger and a camera which captures an image of a near-infrared ray passing through the finger are placed. An optical filter which transmits a near-infrared ray is attached to the camera. Upon biometric authentication, a finger is placed in the path of a near-infrared ray from the light source, and an image of the finger is captured by the camera. Although a near-infrared ray passes through a muscle, fat, bone and so on in vivo, it is absorbed by hemoglobin in the blood and a pigment component such as melanin.
Therefore, although an image which is captured by the camera is shown as white with transmitted light, a blood vessel portion is shown as black. The captured blood vessel pattern image is then checked against registered blood vessel patterns, thereby performing biometric authentication (cf. Japanese Unexamined Patent Application Publication Nos. 2005-312749 and 2005-71118).
There is a growing demand for a smaller and thinner biometric authentication apparatus for carrying out such biometric authentication. As an imaging device which is used in a biometric authentication apparatus, a monocular reduction optical system with the use of reduced imaging of a single or a plurality of lenses has been used. However, there is a limit to the size and thickness reduction for its lens structure.
Thus, there is an attempt to apply a compound-eye optical system which combines a microlens array with a plurality of photoreceptors to a biometric authentication apparatus. An imaging device with a compound-eye optical system is described in Japanese Unexamined Patent Application Publication No. 3-157602, for example.
Japanese Unexamined Patent Application Publication No. 3-157602 discloses an imaging device that includes a microlens array, a photoreceptor portion (image sensor) which is placed opposite to the microlens array, a light shielding layer (light shielding spacer) which is placed between the microlens array and the photoreceptor portion, and a plurality of apertures (translucent holes) having a light absorbing surface on at least part of its wall surface, which are placed between the microlens array and the photoreceptor portion.
FIG. 19 is a vertical sectional view of an imaging device disclosed in Japanese Unexamined Patent Application Publication No. 3-157602. According to the technique disclosed therein, a light shielding layer (light shielding spacer) 20A having a plurality of cylindrical apertures (translucent holes) 30A is placed between a microlens array (lens array plate) 50A and a photoreceptor portion (contact image sensor) 10A. The plurality of cylindrical apertures 30A are formed corresponding to the positions of microlenses (minute lenses) 51A and photoreceptors (photo-detectors) 11A.
A black coating which functions as a light absorbing surface is applied to the inside of each aperture 30A. With such a configuration, the technique disclosed in Japanese Unexamined Patent Application Publication No. 3-157602 can reduce the disadvantages of a compound-eye optical system. Specifically, it prevents the superposition of noise on a received signal at the photoreceptor 11A due to crosstalk, which is, entrance of light from a microlens 51A different from a corresponding microlens 51A into the photoreceptor 11A.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 3-157602 allows suppression of crosstalk from an adjacent microlens 51A. However, the photoreceptor 11A receives light from the range of an area B. An area A is a desired imaging area above the microlens 51A which corresponds to the microlens 51A. The area B includes the area A and it is wider than the area A. Light from an area that excludes the area A from the area B becomes a noise component. Thus, if light from the area B enters the photoreceptor 11A, the quality of a signal which is generated in the photoreceptor 11A is degraded, causing an image on an obtained picture to appear blurred.
Further, if a gap between the microlens array 50A and the photoreceptors 11A is narrowed, the area B becomes wider. This results in further degradation of the quality of a signal from the photoreceptor 11A. This is a serious problem in reducing the thickness of an imaging device.